There is themes.zip in the runtime folder on MWS within a subfolder themes “..\runtime\themes\themes.zip\”.
Inside this ZIP you find xml-files like this.
You can copy FndTheme_Aurora.xml, rename it and change some content in the xml. This needs to be done for some other xmls in subfolders of ThemeResources in the same way.
like <ThemeName Type="System.String">Doppelmayr</ThemeName>
RGB color definitions can be changed for the different screen elements.
I don’t have more documentation on this and I don’t know details how this gets deployed on MWS.